This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in Japanese Patent Application No.2001-380272 filed Dec. 13, 2001.
1. Field of the Invention
The present invention relates to a vehicle power transmission device, and more particularly to a vehicle power transmission device in which a fluid coupling and a wet friction clutch capable of engagement/disengagement control are provided in series.
2. Description of the Related Art
As is illustrated in FIG. 1, the present inventors have developed a new vehicle power transmission device in which a fluid coupling 2 and a wet friction clutch 3 are provided in series at points along a power transmission path which extends from an engine E to a transmission T/M, and in which the wet friction clutch 3 automatically engages and disengages during transmission. In this case, following an operation to put a vehicle in gear when the vehicle is stationary, the clutch is automatically engaged, thereby generating creep. A typical AT automobile is similar in this respect.
If the clutch engages too quickly, clutch engagement shock (so-called garage shock or the like) is produced, and if the clutch engages too slowly, creep generation takes time following the operation to put the vehicle into gear such that the driver does not know when best to step on the accelerator (large time lag). Thus, in order to strike a balance between reducing the engagement time and the occurrence of clutch engagement shock, control is performed such that the clutch is rapidly engaged in the clutch bite region prior to the beginning of clutch connection, and once the clutch begins to connect the engagement speed is switched to low speed and the clutch is slowly connected (slowly engaged).
More specifically, the operating fluid pressure for driving the engagement/disengagement of the clutch is varied in accordance with duty pulses outputted from an electronic control unit (to be referred to as ECU hereinafter), and the clutch is engagement/disengagement controlled thereby. Further, since this control is open control, the ECU outputs duty pulses in accordance with a predetermined program.
As is illustrated by the broken line in FIG. 5, conventional clutch engagement control is performed by first outputting a predetermined starting duty Dstxe2x80x2 from the ECU such that the clutch is engaged to a large extent up to a position near the beginning of clutch connection (this being known as xe2x80x9cone-shot engagement controlxe2x80x9d), then by outputting predetermined slow engagement duties Dkxe2x80x2 from the ECU at predetermined time intervals such that the clutch becomes slowly engaged, and finally, when a predetermined final slow engagement duty Dedxe2x80x2 has been reached, by outputting a full engagement duty Dcxe2x80x2 (=0%) from the ECU such that the clutch becomes fully engaged.
The position at which clutch connection begins, or in other words the torque transmission starting point at which predetermined torque can first be transmitted, will be referred to as the torque point. This torque point is learned by the ECU and used as a reference value for engagement speed switching. The torque point is set as a learning value because of irregularities and individual differences among clutches caused by construction errors, and because the torque point differs from clutch to clutch or from vehicle to vehicle.
In FIG. 5, the torque point is Dlt, and the starting duty Dstxe2x80x2 is typically a value which is slightly further toward the disengagement side than the torque point Dlt. As a general principle, provision is made such that excessive engagement shock is not generated by the one-shot engagement control.
Attention will now be focussed upon a garage shift performed during vehicle start-up. Clutch engagement control similar to that described above is also performed here. FIG. 6 shows the state of creep variation during an operation to put the vehicle in gear directly preceding start-up (when a so-called garage shift is being performed), and also illustrates the state of rotation speed variation of the input side (pump) and output side (turbine) of the fluid coupling. The rotation speed of the input side of the fluid coupling is switched to the engine rotation speed Ne (unbroken line). The rotation speed of the output side of the fluid coupling is switched to the turbine rotation speed Nt (dot/dash line) and may be switched as is to the rotation speed of the clutch input side.
Any time earlier than time t0 indicates a braking operation, neutral gear, and clutch disengagement. At time t0, the gear is fully engaged from the aforementioned state to the start-up level, and clutch engagement control begins. Since the output side of the clutch is damped by a brake from the drive wheel side, slippage of the fluid coupling increases as clutch engagement proceeds, and the pump, which is the input side of the fluid coupling, rotates at a constant idling rotation speed which is equal to the engine rotation speed Ne, whereas the turbine rotation speed Nt gradually drops. By this process, creep gradually increases.
Now focussing on control at the point of half clutch completion, this is conventionally performed using a method in which the full engagement duty Dcxe2x80x2 (=0%) is outputted during the output of the slow engagement duties Dkxe2x80x2 when the predetermined final slow engagement duty Dedxe2x80x2 has been reached, whereupon slow engagement control is switched to full engagement control. In other words, if the outputted duties reach the final slow engagement duty Dedxe2x80x2 at the point of half clutch completion, then the clutch is fully engaged.
When this method is used in reality, however, the clutch transmission torque at the point of half clutch completion may be unstable due to fluid pressure fluctuation or micro fluctuations in the wet friction clutch during control, and consequently the clutch cannot be engaged smoothly. The cause of this may be the moment at the point of half clutch completion when the clutch plates move from kinetic friction to static friction and the relative rotation difference therebetween is small. As a result of this moment, stability cannot be achieved on a micro level, and there is little redundancy with respect to fluctuation.
Problems arise in particular when the half clutch state (or the amount of time of slow engagement control) is protracted: hunting is produced in the turbine rotation due to temperature rises or the like in the clutch; irregular vehicle behavior (such as shock) is produced; and rattling sounds are produced due to collisions among the gears in the transmission.
The present invention has been devised in consideration of these problems, and it is an object thereof to create stability in the clutch transmission torque at the point of half clutch completion during a garage shift upon vehicle start-up, and thereby to engage the clutch smoothly.
The present invention is a vehicle power transmission device in which a fluid coupling and a wet friction clutch are provided in series at points on a power transmission path which extends from an engine to a transmission, and which performs engagement/disengagement control of the clutch by varying the operating fluid pressure used for engagement/disengagement driving the clutch in accordance with duty pulse signals outputted from an electronic control unit, wherein, when a vehicle starts up from a standstill, clutch engagement control is begun at the same time as the transmission is put into gear during a braking operation and a state of clutch disengagement, and a duty pulse signal corresponding to full clutch engagement is outputted from the electronic control unit when the rotation difference on the input and output sides of the fluid coupling reaches or exceeds a predetermined value during engagement control.
In this case it is preferable that this clutch engagement control be such that initially, one-shot engagement control, in which a predetermined starting duty pulse signal is outputted from the electronic control unit, is executed such that the clutch is largely engaged up to the vicinity of the torque point and excessive clutch engagement shock is never produced by taking into consideration irregularities in the torque point, whereupon the process moves to slow engagement control. It is also preferable that this slow engagement control be such that first slow engagement duty pulse signals are outputted from the electronic control unit at predetermined time intervals such that the clutch is engaged more slowly than in the one-shot engagement control, and then second slow engagement duty pulse signals are outputted from the electronic control unit at predetermined time intervals once predetermined conditions have been fulfilled such that the clutch is engaged more slowly than in the first slow engagement.
The present invention is also a clutch control method for performing engagement/disengagement control of a clutch in accordance with duty pulse signals outputted from an electronic control unit, which is used in a vehicle power transmission device in which a fluid coupling and a wet friction clutch are provided in series at points on a power transmission path which extends from an engine to a transmission, wherein, when a vehicle starts up from a standstill, clutch engagement control is begun at the same time as the transmission is put into gear during a braking operation and a state of clutch disengagement, and a duty pulse signal corresponding to full clutch engagement is outputted from the electronic control unit when the rotation difference on the input and output sides of the fluid coupling reaches or exceeds a predetermined value during this engagement control.
The present invention is also a power transmission device in which a fluid coupling and a friction-type clutch capable of performing engagement/disengagement control are provided in series at points on a power transmission path, wherein clutch engagement control is executed when the output side of the clutch has been damped, and full clutch engagement control is proceeded to during the process of clutch engagement control when the rotation difference between the input and output sides of the fluid coupling reaches or exceeds a predetermined value.